Gas hydrates are ice-like non-stoichiometric crystalline compounds. These are cages of water molecules, formed around guest molecules, which are simply called hydrates in gas and oil industries.
The conditions necessary for the formation of hydrates include the presence of water or ice, the presence of a non-polar gas or liquid or a gas or liquid of low polarity and of course proper temperatures and pressures. Water molecules form cages around the guest molecule, as a result of their hydrogen bonding, however they form no chemical bonds with the guest. The gaseous guest molecules are actually compressed and trapped in this porous structure, giving it the potential for storing gas compounds and for their transportation [Sloan, Jr. D., “Fundamental Principles and Applications of Natural Gas Hydrates”, Nature, 246(6964), 353-359 (2003).].
Hydrates of interest in industries, especially in the production and processing of natural gas and oil, are composed of water and guest molecules, such as for example methane, ethane, propane, iso-butane, normal butane, nitrogen, carbon dioxide, hydrogen sulfide and/or hydrogen [Sloan, Jr. D., “Fundamental Principles and Applications of Natural Gas Hydrates”, Nature, 246(6964), 353-359 (2003).]. Other guest species like for example ethylene, N2O, acetylene, vinyl chloride, methane halides, ethane halides, cyclo-propane, methyl mercaptanes, sulfur dioxide, Ir, Ar, Xe, oxygen, trimethylene oxide etc. can also form hydrate clathrates.
Additives having different properties can be used during the formation of gas hydrates. Compounds prohibiting the formation of such structures are so called hydrate inhibitors. One may distinguish between two groups of thermodynamic inhibitors, such as for example methanol, glycols and others that avoid the formation of hydrates by shifting the phase diagram of the system and kinetic inhibitors, such as for example polyvinyl pyrrolidone, which postpone the formation of hydrates up to some days.
Other types of additives having a rather opposite effects are also known and are so called hydrate promoters, such as for example sodium dodecyl sulfate promoting the formation of hydrates.
The major problem with the application of natural gas as a fuel is its transportation, because of its low density, i.e. small amounts of natural gas have high volumes. One solution to this problem is the high-pressure storage of natural gas, which is performed in two distinct methods of liquefied natural gas (LNG) and compressed natural gas (CNG), depending on the transportation systems used. LNG is very expensive from a process equipment and transportation equipment point of view. CNG on the other hand, is not a suitable method for gas transportation due to the high volume of the compressed gas.
Conversion of methane to methanol, which is a liquid and easily transportable fuel, might be an alternative but it is not a proper method due to the high costs and the required operations loosing of up to 47% of the heat value of natural gas.
The production of hydrates for storing the hydrocarbon gases can be another alternative. This will be a less costly method with higher levels of safety and will require less energy and equipment for forming the hydrate and dissociating it at the final destination. It is also safer than the traditional methods of LNG and CNG, which always accompany the risk of explosion, especially in the case of accidents.
The major problems of using hydrates for the purpose of making gases transportable is the high pressure necessary for forming, storing, and transporting the hydrates. Numerous efforts have been made to solve this problem and one solution proposed is the application of slurry hydrates that are formed by gradual addition of gases to water, suggested in U.S. Pat. No. 6,082,118. This method however, suffers drawbacks including a low final gas content that make it expensive and not economic.
Stern L. A, et al (Energy and Fuels 15(2), 2001, 499-501) and Tse (J. Supramol. Chem., 2, 2002, 467-472) reported that decreasing the pressure over the hydrates leads to their decomposition, and because this is an endothermic process, the molten layer of the hydrate converts to ice, protecting the remaining hydrate, which is entitled the self-preservation phenomenon. Stern L. A. paid specific attention to the stabilization of methane hydrates in 50-75° over the equilibrium temperature (193 K) and under atmospheric pressure, using pressure release methods.
Satoshi T., (J. Phys. Chem. A, 105(42), 2001, 9756-9759) reported that the bigger the size of the hydrate particles is, the higher the probability of keeping hydrates under higher temperatures will be. For example, they mentioned that for hydrate particles of 1000-1400 μm in dimension, a low gas content of only about 20% (v/v) of methane can be achieved in 263 K and 1 atm.
Stern et al, (Energy & Fuels, 15 (2), 2001, 499-501), Tse, et al., (J Supramol. Chem., 2, 2002, 467-472), and Kush et al., (Phys Chem. Chem. Phys., 6(27), 2004, 4917-4920) suggested using the self-preservation property of hydrates. This method suffers disadvantages, like low stability, conversion of hydrate to ice, and their low gas content.
U.S. Pat. No. 3,975,167 describes a method for forming hydrates by a special process and apparatus, which provide the temperature and pressure for the formation the hydrate in a suitable depth of the sea. According to this invention hydrates are formed using proper cooling systems and through providing the required pressure by choosing the proper depth in water. The gas is released in the destination by bringing the hydrate to the surface and heating it. However, also expensive equipment is necessary for such processes.
U.S. Pat. No. 5,536,893 describes a method for forming and transportation of hydrates. This patent discloses the details of the system and process of production of hydrates from water and gas. The method is based on spraying water and cooled gas, which is followed by hydrate formation, its removal from the reactor, its agglomeration, increasing its density, saturation of its pores with the gas and finally its storage or transportation.
But this method is used under very difficult-to-achieve adiabatic conditions. The hydrate storage is performed outside the thermodynamic hydrate stabilization area shown in FIG. 1 on the same text (−10-150° C. and atmospheric pressure), which naturally leads to the ice-formation on the surface of the hydrate phase, and the reduction of gas storage capacity of the formed hydrates, according to hydrate-phase thermodynamic principles.
The recovered gas, in addition, is only 20-70% of what is initially stored, which is not directly mentioned in the patent, but is actually expected to be very low due to the inevitable hydrate storage conditions.
U.S. Pat. No. 6,082,118 storage and transportation of slurry hydrates suspended in liquid hydrocarbons under metastable conditions are disclosed. The hydrates formed in this invention, however, have low gas contents.
The stability of hydrates is defined by their inherent phase diagrams. Gas hydrates have high stabilities at high pressures (e.g. 150 bar) and low temperatures (e.g. 4° C.). It should also be noted that the pressure should be adjusted with using the same gases as for the desired hydrates in order not to disrupt the thermodynamic equilibrium of the existing phases. Given that the phase boundary curves of gas hydrates are of exponential nature, the so-called hydrate formation zone is much wider at higher pressures.
For instance, taking 0° C. as the reference temperature the methane hydrates formed under a pressure of 100 bar will be stable in a temperature range of from 0-13° C., while if the pressure is reduced to 50 bar, methane hydrate will be stable only in the range of 0 to 5.8° C., as described in FIG. 1.
Temperatures below 0° C. bear the risk of ice-formation, which leads to the release of the gases stored in the primary hydrate structure. The advantage of high pressures is that by adjusting the system composition in a way that water is the limiting reactant, all of the water can be converted to hydrates that are highly saturated with gas molecules. But even at high pressures (e.g. 150 bar for CH4), and in particular at low pressures, the hydrate pores are not filled with the gas molecules, and the gas content of the hydrate is not high. Additional tests also show that hydrates formed through the above-mentioned methods known in the art suffer disadvantages, like low gas-contents, low mechanical stability, low yield, and long equilibrium times in the hydrate formation process.
On the other hand, although the formation of hydrates at high pressures and low temperatures (e.g. 250 bar, and 4° C.) is favorable, transportation of gases in hydrates under such high pressures can be highly dangerous.
Many efforts have been made to use the hydrate self-preservation phenomenon in order to make it possible to store hydrates under milder conditions, like atmospheric pressure up to 30 bar. In order for the storage pressure to be 1 bar, the system temperature must be reduced to −20° C. or lower (e.g. −40° C.), which is very costly, and also causes the disadvantage of the ice-formation, as well as the fact that the formed hydrates will have low gas contents.
In general the storage pressure and gas content of hydrates are important factors in storage conditions. It is noteworthy that the higher the gas content of a hydrate is, the faster its dissociation will be.
One would also realize that, although the hydrate self-preservation is a fully understood phenomenon, there is no data available on the long-term stabilization of hydrates in or out of the thermodynamic stability conditions of hydrates.
Hydrates start to change to ice at temperatures below zero (0° C.), especially between 0 to minus 33° C. This has been proved by neutron diffraction spectroscopy [Kush. W F, et al, Phys. Chem. Phys. 6(21), 2004, 4917-49201]. The ice particles formed in the temperature range of from 0 to minus 33° C. are of hexagonal (Ih) crystalline structure, and their agglomeration prohibits the gas from leaving the hydrate structure. Below −33° C., cubic ice (IC) is formed, which has far less agglomeration, and hence a far less ability of blocking the gases, and the hydrates are hence gradually dissociated.
There exists a strong need to prepare hydrates with high gas contents and achieve good stability allowing their transportation at conditions that are relatively “mild”, such as for example low pressure. Also, further drawbacks described above according to the prior art should be resolved.